From d3e42beb106479eae7c234bb0aeacce92661ca7a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=A9r=C3=A9mie=20Galarneau?=
 <jeremie.galarneau@efficios.com>
Date: Fri, 7 Dec 2018 17:06:37 -0500
Subject: [PATCH 08/15] Fix: don't wait after completion of job batch if work
 queue is empty
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On completion of a batch of jobs from the work queue, a wait of 10
ms (using poll()) is performed if there is no work present in the
work queue before waiting on its futex.

The work queue thread's structure is inspired by the call-rcu thread.
In the context of the call-rcu thread, my understanding is that the
intention is to ensure that the thread does not continuously wake-up
to process a single queued item. This is fine as an application should
not wait for a call-rcu job to be executed (or at least I don't see a
use-case for that).

In the context of the work queue, waiting for more work to be available
artificially slows down the execution of work on which an application
may wait.

I have observed a case where LTTng's session daemon's shutdown is
takes around 4 seconds as a large number of cds_lfht objects are
destroyed. Removing the wait reduces the duration of this phase of the
shut-down to almost ~10ms.

If a workqueue user even need the explicit delay for batching (e.g. if
a call-rcu implementation would ever use the workqueue worker thread),
it can add it within the worker_before_wait_fct callback received as
argument from workqueue creation.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 src/workqueue.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/workqueue.c b/src/workqueue.c
index 6707ffe..0b1a9ea 100644
--- a/src/workqueue.c
+++ b/src/workqueue.c
@@ -238,7 +238,6 @@ static void *workqueue_thread(void *arg)
 			if (cds_wfcq_empty(&workqueue->cbs_head,
 					&workqueue->cbs_tail)) {
 				futex_wait(&workqueue->futex);
-				(void) poll(NULL, 0, 10);
 				uatomic_dec(&workqueue->futex);
 				/*
 				 * Decrement futex before reading
-- 
2.19.1